1. Field of the Invention
The present invention relates to turbines, and more particularly, to gas or steam turbines.
2. Description of the Prior Art
Rotor blades in a turbine, especially in smaller turbines, are normally unshrouded. The root of the blade is fixed to a hub, but the outer radial end or tip of the turbine rotor blade is free. A liner of cylindrical shape is normally provided in the stationary housing acting as a shroud for the blades. However, since the shroud liner is stationary, it is necessary to leave a gap or tolerance between the tip of the blade and the shroud liner so as to avoid rubbing. It is necessary to provide a sufficient gap between the tip and the shroud in order to provide for the differences in the expansion of the respective metal components. A minimum practical clearance has been found to be 1% of the blade height. However, when such gaps are provided, the gases on the high pressure side of the blade tend to leak over the tip of the blade at a relatively higher velocity than the rotating velocity of the blade to thereby interfere with the low pressure side of the blade and deteriorate the flow pattern of the gases on the low pressure side increasing, for instance, separation of the gas flow from the surface of the blade on the low pressure side thereof. Furthermore, the gap causes some of the gas to bypass the rotor, and thus not contribute to the work, since it is not turning relative to the rotor blades.
Various improvements and developments have been made to reduce the actual gap between the tip of the blade and the shroud liner as, for example, the ablative seal described in U.S. Pat. No. 3,836,156, issued Sept. 17, 1974, Hector B. Dunthorne, and assigned to the applicant. This helps to reduce the size of the gap, but does not eliminate the tip leakage. One common method for reducing the amount of tip leakage flow, as well as its disruptive influence on the flow on blade low pressure surface, is to equip the rotor tip with a shroud.
Each blade has at its tip a segment of a ring, such that when assembled in a rotor disc, these shroud segments form a continuous ring which prevents flow from within rotor blade passages from leaking around the blade tips. Due to the necessary tip clearance, some flow will still leak past the rotor blades, but at least it does not disrupt the mainstream flow on blade low pressure surfaces.
However, any such shroud causes a relatively large amount of metal to be added to rotor blade tips. This is most undesirable in the case of first stage turbine blades, because of the high gas temperature at the exit of the combustion chamber immediately preceding the turbine. The additional mass of the blade created at the tip causes the centrifugal stresses in the rotor blades to be substantially increased, with the result of a much reduced rotor blade life; while there are ways to alleviate this problem, such as a reduced gas temperature, or blade roots with very large metal areas, hence thick discs, considerations of overall engine efficiency and weight usually dictate the elimination of such shrouds on first stage blades.